The present invention relates to a method for the purification of silicon or, more particularly, to an inexpensive and efficient method for the purification of low-grade silicon to obtain silicon having purity suitable and sufficient for use as an element of solar cells.
As is known, the most serious problems of global concern in the near future would include the problem of global warming-up as a consequence of the greenhouse effect due to accumulation of carbon dioxide gas produced by the combustion of various carbon-containing fuels, such as petroleums and coals, in the atmosphere and the problem of exhaustion of fossil fuels as a non-reproducible resource. In this regard, great efforts are being made for the research and development works of a substitute energy source which is free from exhaustion of resources and free from emission of carbon dioxide gas, of which the most promising and highlighted in recent years is the technology of solar power generation by the utilization of the solar energy as a clean and non-exhaustible energy source. Practically, the solar power generation is conducted by using so-called solar cells with solar panels of high-purity silicon as the cell elements capable of directly converting the solar energy into electric energy.
The largest factor which retards development of the solar power generating system and limits prevalence of the system is the expensiveness of high-purity silicon materials used for the solar cell elements so that various attempts and proposals have been made for the cost reduction in the production of high-purity silicon along with an improvement in the reliability of the solar power generating system.
Since the amount of demand for the high-purity silicon as a base material of solar cells still remains at a low level, the silicon material currently employed in the manufacture of solar cells is not a product from a process specialized therefor but mostly supplied by diversion of the silicon materials produced for other applications. For example, crystalline silicon as the base material of solar cells is supplied from the manufacturing process of semiconductor silicon devices such as an intermediate product and scraps such as conical cutoff debris of tops and tails taken from single crystal rods of semiconductor silicon as grown, remnant melt of silicon in the crucible after Czochralski growing of a single crystal silicon rod therefrom, unacceptable crystals, broken silicon wafers and so on.
While the quantity of high-purity silicon materials for solar cells currently available from the above mentioned diversion routes is naturally limited, the estimated demand of solar cell-grade silicon for fullscale development of the solar power generating system is so great that development of a process specialized for the production of solar cell-grade high-purity silicon is very important and urgent in order that the electric power cost by the solar power generating system can be competitive with the current cost of the electric energy by other traditional power generating systems.
As to the quality of the high-purity silicon for solar cell use, which has the P-type electroconductivity with a resistivity of 1 ohm.cm or higher, the purity thereof relative to the content of silicon is relatively low to be about 6-nines in % or somewhat higher as compared with the purity of the semiconductor-grade high-purity silicon which is said to be 11-nines or higher. This difference in the quality of the high-purity silicon materials between different applications is suggestive that there is no reason which may justify the diversion use of the semiconductor-grade high-purity silicon materials for the material of solar cells because the production cost of the semiconductor-grade high-purity silicon having overly quality is necessarily very high in order to enable a solar cell prepared therefrom to be competitive in costs with other power generating systems.
In view of the above described quality requirement for the solar cell-grade high-purity silicon, attempts have been made to upgrade metallurgical-grade metallic silicon, which has a silicon purity of 98% or higher and is under industrial mass production at low costs in the steel industry, to give solar cell-grade silicon having the above mentioned quality. The metallurgical-grade metallic silicon is produced by reducing silica stone as a mineral occurring in nature with a carbonaceous material such as cokes as the reducing agent so that the product unavoidably contains various impurities including boron and phosphorus which may have a great influence on the performance of the solar cells prepared from a silicon material containing these impurities. No efficient method, however, has been developed to remove these impurity elements from the metallurgical-grade metallic silicon to such an extent that the silicon can be used as the material of solar cells.
The annual production of the above mentioned silicon scraps is also quite large including the cutting dusts and chips generated in the slicing works of silicon single crystal rods for the preparation of semiconductor silicon wafers. Accordingly, it is also very advantageous to recycle and re-use these silicon scraps as the starting material in the production of solar cell-grade silicon materials at low production costs. Although these silicon scraps usually do not contain metallic impurities such as iron, calcium, aluminum and the like, the impurity contents of boron and phosphorus, which can be removed only with great difficulties, are also quite high as in the metallurgical-grade metallic silicon.
As a trend in recent years, moreover, many of the semiconductor memories are manufactured from semiconductor silicon wafers taken from a single crystal silicon rod having a P-type resistivity of 1 ohm.cm or lower and provided with a high-resistivity thin film of silicon by the epitaxial method on the wafer surface. As a consequence of this trend, an increasingly large portion of the silicon scraps produced in the semiconductor processes is occupied by low-resistivity silicon materials so that such a low-resistivity silicon scrap can be used as the starting material for the preparation of a solar cell-grade silicon material only by the combined use of a high-resistivity silicon scrap or freshly obtained high-resistivity polycrystalline silicon in such a proportion that the product as a combination thereof may have a resistivity of 1 ohm.cm or higher. This method, however, is also not practical to ensure production of solar-grade silicon at low costs due to the use of the relatively expensive resistivity-adjustment adjuvant silicon.
On the other hand, re-use of N-type silicon scraps containing phosphorus as the dopant is also under investigations but the difficulty encountered in the removal of the phosphorus impurity is the same as in the removal of the boron impurity.
In view of the above described situations, it is eagerly desired to develop an efficient and reliable purification method of low-grade silicon materials by which metallic impurities as well as boron and phosphorus could be removed from metallurgical-grade metallic silicon or silicon scraps.
As a method for the removal of boron and phosphorus from a low-grade silicon material, a proposal is made in Japanese Patent Kokai 4-16504 and 64-56311 in which the starting metallurgical-grade metallic silicon is melted under a reduced pressure in a direct-current arc furnace or electron beam furnace followed by unidirectional solidification. Further, Japanese Patent Kokai 9-48606 discloses a method in which purification of metallic silicon is performed by the combination of electron beam melting and ion beam irradiation.
The former of the above described methods, however, is not quite efficient for the removal of the impurities such as boron and phosphorus of which the distribution coefficient between solid and liquid phases of silicon is close to 1. Accordingly, an improvement of the method is proposed in which an oxidizing gas is intermixed with the atmospheric gas or a treatment with a calcium-based flux is undertaken though not with very promising results. The latter of the above described methods, on the other hand, is not advantageous in practicing and not suitable for mass production of solar cell-grade silicon at low costs because the technique to conduct the method is very elaborate and the investment for the apparatuses therefor is very great.